5 Water Chemistry and Biochemistry 125Energy from the sun captured by plants and or-
ganisms converts carbon dioxide and water to high-
energy carbohydrates,
6CO 2 12H 2 O →(CH 2 O) 6 6H 2 O 6O 2
The stars () indicate the oxygen atoms from water
released as oxygen gas (O 2 ). Still, this is an over-
simplified equation for photosynthesis, but we do
not have room to dig any deeper. The product
(CH 2 O) 6 is a hexose, a six-carbon simple sugar, or
monosaccharide,that can be fructose, glucose, or
another simple sugar. Glucose is the most familiar
simple sugar, and its most common structure is a
cyclic structure of the chair form. In glucose, all the
OH groups around the ring are at the equatorial
positions, whereas the small H atoms are at the axial
locations (Fig. 5.15). With so many OH groups per
molecule, glucose molecules are able to form sever-
al hydrogen bonds with water molecules, and thus
most monosaccharides are soluble in water.
The disaccharidessucrose, maltose, and lactose
have two simple sugars linked together, whereas
starch and fiber are polymers of many glucose units.
A disaccharide is formed when two OH groups of
separate monosaccharides react to form an –O– link,
called a glycosidic bond, after losing a water mole-
cule.
C 6 H 12 O 6 C 6 H 12 O 6 C 6 H 11 O 5 –O–C 6 H 11 O 5
H 2 O
Disaccharides are soluble in water due to their abil-
ity to form many hydrogen bonds.
Plants and animals store glucose as long-chain
polysaccharidesin starchand glycogen, respec-
tively, for energy. Starch is divided into amylose and
amylopectin. Amyloseconsists of linear chains,
whereas amylopectinhas branched chains. Due to
the many interchain hydrogen bonds in starch, hy-
drogen bonding to water molecules develops slowly.
Small starch molecules are soluble in water. Sus-
pensions of large starch molecules thicken soup and
gravy, and starch is added to food for desirable tex-
ture and appearance. Water increases the molecular
mobility of starch, and starch slows the movement
of water molecules. Food processors are interested
in a quantitative relationship between water and
starch and the viscosity of the suspension.
Glycogen, animal starch, is easily hydrolyzed to
yield glucose, which provides energy when re-
quired. In the hydrolysis of polysaccharides, water
molecules react at the glycosidic links. Certain
enzymes catalyze this reaction, releasing glucose
units one by one from the end of a chain or branch.
Polysaccharide chains in cellulose are very long,
7000 to 15,000 monosaccharides, and interchain
hydrogen bonds bind them into fibers, which further
stack up through interfiber hydrogen bonds. Many
interchain hydrogen bonds make the penetration of
water molecules between chains a time-consuming
process. Heating speeds up the process.WATER, MINERALS, ANDVITAMINSMost minerals are salts or electrolytes. These are
usually ingested as aqueous solutions of elec-
trolytes, discussed earlier. Ions (Ca^2 , Mg^2 , Na,
K, Fe^2 , Zn^2 , Cu^2 , Mn^2 , Cl, I, S2-, Se2-,
H 2 PO 4 , etc.) present in natural water are leached
from the ground. Some of them are also present in
food, because they are essential nutrients for plants
and animals that we use as food. A balance of elec-
trolytes in body fluid must be maintained. Other-
wise, shock or fainting may develop. For example,
the drinking water used by sweating athletes con-
tains the proper amount of minerals. In food, miner-
al absorption by the body may be affected by the
presence of other molecules. For example, vitamin
D helps the absorption of calcium ions.
Small amounts of a group of organic compounds
not synthesized by humans, but essential to life, are
called vitamins; their biochemistry is very compli-
cated and interesting; many interact with enzymes,Figure 5.15.Chair form cyclic structure of glucose
C 6 H 12 O 6. For glucose, all the OH groups are in the
equatorial position, and these are possible H-donors for
hydrogen bonding with water molecules. They are also
possible sites to link to other hexoses.